1. Field of The Invention
The present invention relates to improvement of an ultrasonic beam in the elevation direction of an ultrasonic transducer and more specifically to the shading of an electromechanical coupling coefficient in the elevation direction of a piezoelectric vibrator of an ultrasonic transducer.
2. Description of The Related Art
In view of improving an ultrasonic beam, namely reducing side lobe level of ultrasonic beam, polarization of arranged vibrators forming an ultrasonic transducer as a piezoelectric material has been lowered or reduced in mass toward the end portion of the transducer from the center in the direction orthogonally crossing the arrangement direction of vibrators (namely, in the elevation direction of ultrasonic transducer, and in the elevation direction of a probe).
FIG. 1(a) indicates an example of such a structure. In this figure, the vertical axis indicates an electro-mechanical coupling coefficient, while the horizontal axis indicates the direction orthogonally crossing the arrangement direction of vibrators forming an ultrasonic transducer as the piezoelectric material (namely, elevation direction of ultrasonic transducer, and elevation direction of probe). In FIG. 1(a), the polarized distribution of coupling coefficient is similar to the Gaussian function. Namely, polarization is carried out so that the distribution of electromechanical coupling coefficient kt (hereinafter referred to as coupling coefficient) of vibrators arranged is gradually reduced as the polarization goes to the end portion of the transducer from the center. An acoustic pressure from the ultrasonic transducer in such polarization is shown in FIGS. 3 (a), (b). FIG. 3(a) indicates the ultrasonic beam irradiating direction on the horizontal axis and elevation direction of arranged vibrators (direction orthogonally crossing the arrangement direction) on the vertical axis. The acoustic beam profiles in the graph respectively show +20 dB, +10 dB, -10 dB, -20 dB. FIG. 3(b) indicates a distribution of an acoustic pressure in the area separated by 140 mm from the arranged vibrators, namley the sectional view of the acoustic pressure at the point corresponding to elevation direction of arranged vibrators separated by 140 mm from the arranged vibrators in FIG. 3(a). The vertical axis of FIG. 3(b) indicates acoustic pressure, while the horizontal axis indicates the elevation direction (direction orthogonally crossing the arrangement) of the arranged vibrators.
FIG. 1(b) indicates an example where the polarization of arranged vibrators is uniform for the elevation direction (without shading). The acoustic pressure graph of acoustic beam profile in this case is shown in FIGS. 4(a), and 4(b). The graphs of FIGS. 4(a), and 4(b) indicate just like FIG. 3(a) and 3(b).
In comparison of these graphs, it is understood that the side lobe level is high when the coupling coefficient is not shaded (comparison in FIG. 3(b) and FIG. 4(b)) and that the-beam is not converged (comparison in FIG. 3(a) and FIG. 4(a)).
As the method (a) for changing polarization of arranged vibrators, a method has been proposed by D.K. Hsu in IEEE shown in FIG. 2 on Oct. 9, 1989 ("IEEE 1989 ULTRASONIC SYMPOSIUM AND SHORT COURSES, PROGRAM AND ABSTRACTS NON-UNIFORMLY POLED GAUSSIAN BESSEL FUNCTION TRANSDUCERS"). First, a piezoceramics 102, which is sufficiently thicker than the desired elevation and has the spherical recessed area at a single side, is manufactured. Next, an Ar/Cr film 105 is evaporated to or placed on both sides of piezoelectric ceramics. A spherical electrode 101 matching with the shape of curvature of the recessed area is provided to the spherically arcuated surface of the ceramic and a flat electrode 104 is provided in the opposite side to the spherically arcuated surface for polarization. The ceramic is polarized. Thereafter, a flat piezoelectric ceramic can be obtained by polishing or cutting the material to the determined elevation t. Thereby, the coupling coefficient can gradually be reduced from the center of the piezoelectric material to the end portion and amplitude shading can be realized.
As the other method, Published Japanese Patent No. 24479/1989 "Linear Phased Array Ultrasonic Transducer" proposes four additional methods; (b) a method where polarization is carried out by applying a high voltage pulse of long duration to a material and thereafter a low voltage pulse is applied for monitoring polarization of the element; (c) a method where a nonuniform high voltage polarization field is applied to a piezoelectric ceramic plate so that the field becomes maximum at the center of the array and the field is lowered or reduced in intensity at the both end portions and in this case, the polarization apparatus is formed by spherically arcuated plate provided with a dielectric material at both end portions or (d) a method similar to method (c) where the polarization apparatus is formed by a flat resistance material to which a voltage is applied to the side where the piezoelectric ceramic is provided; and (e) a method where a piezoelectric material is polarized so that the coupling coefficient becomes uniform, thereafter a temperature gradient is applied to the piezoelectric ceramic by heating both end portions of piezoelectric material and cooling the center. As a result, the polarization of piezoelectric ceramic is stably and uniformly polarized and then reduced adequately depending on the position thereof.
In the methods (a)-(e) explained above, for the shading function, a function which becomes high in the center and becomes low at both end portions, for example, the continuous function such as square cosine (Y=cos.sup.2 (X)), Humming function or Gaussian function, etc. is used. Therefore, the surface of piezoelectric ceramics must have the continuous voltage distribution depending on the function at the time of polarization. In this case, the following problems are generated in each method.
In the case where the ceramic is formed by the method (a) proposed by D.K. Hsu, first it is difficult to provide or produce the spherically arcuated surface in the ceramic 1. Second, it is also difficult to provide a spherical electrode to the spherically arcuated surface. Third, unwanted portion is cut out after polarization and polished up to the desired thickness. They require more steps than those in the uniform polarization. As explained above, manufacture is difficult and more steps are required.
The method (b) of applying high voltage pulse also requires more period and steps because the high voltage pulse is repeatedly applied while the result is monitored for each application of pulse.
In the method (c) using a dielectric material, the surface of the piezoelectric ceramic must be brought in contact with high accuracy to the surface of dielectric material for the polarization. Namely, it is thought that polarization is interfered or disturbed due to very small nonuniformity and small size of dust or other particles, or warpage of ceramics and dielectric material, etc.
In case a resistance material is used as proposed in the method (d), the surface of resistance material must be brought in contact with high accuracy to the surface of ceramics just like the case where dielectric material is used.
In the method (e) where temperature gradient is applied to the material, in the arrangement direction, it may be thought that polarization at the end portion is not reduced more than the center, comparison between the center and end portions because the more quantity of heat is released from the end portion. Namely, it is difficult to form uniform polarization to all arranged vibrators in the arrangement direction. Moreover, since a constant temperature gradient must be maintained for a certain long period, control becomes difficult and more steps are required.
As explained above, it is very difficult to manufacture the vibrators to give distribution of polarization intensity depending on the continuous function.